Much of the machinery which has been developed over the years is driven by some form of prime mover. Thus efficient power transfer mechanisms are necessary for coupling the prime mover to the driven apparatus. Such power transfer mechanisms range from a simple pair of pulleys and a drive belt to rather complex gearing arrangements. Regardless of the design of the power transfer mechanism, slippage due to a frozen or sluggish driven apparatus usually results in damage to the power transfer mechanism and to the prime mover.
One example of an apparatus which must be coupled to a prime mover is the compressor used in automotive air conditioning systems. Such compressors are usually coupled to the automotive engine by a drive belt through an electromagnetic clutch. If the compressor should freeze (lock) or become difficult to turn while the clutch is engaged, the belt will slip. Slippage causes rapid wear and early breakage of the belt. In some situations, unexpected breakage of a drive belt could have serious results.
For the above reasons, there are systems known in the prior art which detect the rate of rotational speed of a prime mover and a driven apparatus for the purpose of determining whether there is slippage in the power transfer mechanism, for example, as shown in Japanese Patent Publication No. 57-35181. When slippage beyond a set amount is detected, the prime mover is shut down or the driven apparatus is disconnected from the prime mover. Because such devices must measure the rotational speed of both the prime mover and the driven apparatus and then calculate the rate of rotational speed for each, they are rather complicated in construction and quite expensive to manufacture.